1. Field of the Invention
The present invention relates to a light emitting diode used in LED display, back light source, traffic signal, trailway signal, illuminating switch, indicator, etc. More particularly, it relates to a light emitting device (LED) comprising a phosphor, which converts the wavelength of light emitted by a light emitting component and emits light, and a display device using the light emitting device.
2. Description of Related Art
A light emitting diode is compact and emits light of clear color with high efficiency. It is also free from such a trouble as burn-out and has good initial drive characteristic, high vibration resistance and durability to endure repetitive ON/OFF operations, because it is a semiconductor element. Thus it has been used widely in such applications as various indicators and various light sources. Recently light emitting diodes for RGB (red, green and blue) colors having ultra-high luminance and high efficiency have been developed, and large screen LED displays using these light emitting diodes have been put into use. The LED display can be operated with less power and has such good characteristics as light weight and long life, and is therefore expected to be more widely used in the future.
Recently, various attempts have been made to make white light sources by using light emitting diodes. Because the light emitting diode has a favorable emission spectrum to generate monochromatic light, making a light source for white light requires it to arrange three light emitting components of R, G and B closely to each other while diffusing and mixing the light emitted by them. When generating white light with such an arrangement, there has been such a problem that white light of the desired tone cannot be generated due to variations in the tone, luminance and other factors of the light emitting component. Also when the light emitting components are made of different materials, electric power required for driving differs from one light emitting diode to another, making it necessary to apply different voltages different light emitting components, which leads to complex drive circuit. Moreover, because the light emitting components are semiconductor light emitting components, color tone is subject to variation due to the difference in temperature characteristics, chronological changes and operating environment, or unevenness in color may be caused due to failure in uniformly mixing the light emitted by the light emitting components. Thus light emitting diodes are effective as light emitting devices for generating individual colors, although a satisfactory light source capable of emitting white light by using light emitting components has not been obtained so far.
In order to solve these problems, the present applicant previously developed light emitting diodes which convert the color of light, which is emitted by light emitting components, by means of a fluorescent material disclosed in Japanese Patent Kokai Nos. 5-152609, 7-99345, 7-176794 and 8-7614. The light emitting diodes disclosed in these publications are such that, by using light emitting components of one kind, are capable of generating light of white and other colors, and are constituted as follows.
The light emitting diode disclosed in the above gazettes are made by mounting a light emitting component, having a large energy band gap of light emitting layer, in a cup provided at the tip of a lead frame, and having a fluorescent material that absorbs light emitted by the light emitting component and emits light of a wavelength different from that of the absorbed light (wavelength conversion), contained in a resin mold which covers the light emitting component.
The light emitting diode disclosed as described above capable of emitting white light by mixing the light of a plurality of sources can be made by using a light emitting component capable of emitting blue light and molding the light emitting component with a resin including a fluorescent material that absorbs the light emitted by the blue light emitting diode and emits yellowish light.
However, conventional light emitting diodes have such problems as deterioration of the fluorescent material leading to color tone deviation and darkening of the fluorescent material resulting in lowered efficiency of extracting light. Darkening here refers to, in the case of using an inorganic fluorescent material such as (Cd, Zn)S fluorescent material, for example, part of metal elements constituting the fluorescent material precipitate or change their properties leading to coloration, or, in the case of using an organic fluorescent material, coloration due to breakage of double bond in the molecule. Especially when a light emitting component made of a semiconductor having a high energy band gap is used to improve the conversion efficiency of the fluorescent material (that is, energy of light emitted by the semiconductor is increased and number of photons having energies above a threshold which can be absorbed by the fluorescent material increases, resulting in more light being absorbed), or the quantity of fluorescent material consumption is decreased (that is, the fluorescent material is irradiated with relatively higher energy), light energy absorbed by the fluorescent material inevitably increases resulting in more significant degradation of the fluorescent material. Use of the light emitting component with higher intensity of light emission for an extended period of time causes further more significant degradation of the fluorescent material.
Also the fluorescent material provided in the vicinity of the light emitting component may be exposed to a high temperature such as rising temperature of the light emitting component and heat transmitted from the external environment (for example, sunlight in case the device is used outdoors).
Further, some fluorescent materials are subject to accelerated deterioration due to combination of moisture entered from the outside or introduced during the production process, the light and heat transmitted from the light emitting component.
When it comes to an organic dye of ionic property, direct current electric field in the vicinity of the chip may cause electrophoresis, resulting in a change in the color tone.
Thus, an object of the present invention is to solve the problems described above and provide a light emitting device which experiences only extremely low degrees of deterioration in emission light intensity, light emission efficiency and color shift over a long time of use with high luminance.
The present applicant completed the present invention through researches based on the assumption that a light emitting device having a light emitting component and a fluorescent material must meet the following requirements to achieve the above-mentioned object.
(1) The light emitting component must be capable of emitting light of high luminance with light emitting characteristic which is stable over a long time of use.
(2) The fluorescent material being provided in the vicinity of the high-luminance light emitting component, must show excellent resistance against light and heat so that the properties thereof do not change even when used over an extended period of time while being exposed to light of high intensity emitted by the light emitting component (particularly the fluorescent material provided in the vicinity of the light emitting component is exposed to light of a radiation intensity as high as about 30 to 40 times that of sunlight according to our estimate, and is required to have more durability against light as light emitting component of higher luminance is used).
(3) With regard to the relationship with the light emitting component, the fluorescent material must be capable of absorbing with high efficiency the light of high monochromaticity emitted by the light emitting component and emitting light of a wavelength different from that of the light emitted by the light emitting component.
Thus the present invention provides a light emitting device, comprising a light emitting component and a phosphor capable of absorbing a part of light emitted by the light emitting component and emitting light of wavelength different from that of the absorbed light; wherein said light emitting component comprises a nitride compound semiconductor represented by the formula: IniGajAlkN where 0xe2x89xa6i, 0xe2x89xa6j, 0xe2x89xa6k and i+j+k=1) and said phosphor contains a garnet fluorescent material comprising at least one element selected from the group consisting of Y, Lu, Sc, La, Gd and Sm, and at least one element selected from the group consisting of Al, Ga and In, and being activated with cerium.
The nitride compound semiconductor (generally represented by chemical formula IniGajAlkN where 0xe2x89xa6i, 0xe2x89xa6j, 0xe2x89xa6k and i+j+k=1) mentioned above contains various materials including InGaN and GaN doped with various impurities.
The phosphor mentioned above contains various materials defined as described above, including Y3Al5O12:Ce and Gd3In5O12:Ce.
Because the light emitting device of the present invention uses the light emitting component made of a nitride compound semiconductor capable of emitting light with high luminance, the light emitting device is capable of emitting light with high luminance. Also the phosphor used in the light emitting device has excellent resistance against light so that the fluorescent properties thereof experience less change even when used over an extended period of time while being exposed to light of high intensity. This makes it possible to reduce the degradation of characteristics during long period of use and reduce deterioration due to light of high intensity emitted by the light emitting component as well as extraneous light (sunlight including ultraviolet light, etc.) during outdoor use, thereby to provide a light emitting device which experiences extremely less color shift and less luminance decrease. The light emitting device of the present invention can also be used in such applications that require response speeds as high as 120 nsec., for example, because the phosphor used therein allows after glow only for a short period of time.
The phosphor used in the light emitting diode of the present invention preferably contains an yttrium-aluminum-garnet fluorescent material that contains Y and Al, which enables it to increase the luminance of the light emitting device.
In the light emitting device of the present invention, the phosphor may be a fluorescent material represented by a general formula (Re1xe2x88x92rSmr)3(Al1xe2x88x92sGas)5O12:Ce, where 0xe2x89xa6r less than 1 and 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y and Gd, in which case good characteristics can be obtained similarly to the case where the yttrium-aluminum-garnet fluorescent material is used.
Also in the light emitting device of the present invention, it is preferable, for the purpose of reducing the temperature dependence of light emission characteristics (wavelength of emitted light, intensity of light emission, etc.), to use a fluorescent material represented by a general formula (Y1xe2x88x92pxe2x88x92qxe2x88x92rGdpCeqSmr)3(Al1xe2x88x92sGas)5O12 as the phosphor, where 0xe2x89xa6pxe2x89xa60.8, 0.003xe2x89xa6qxe2x89xa60.2, 0.0003xe2x89xa6rxe2x89xa60.08 and 0xe2x89xa6sxe2x89xa61.
Also in the light emitting device of the present invention, the phosphor may contain two or more yttrium-aluminum-garnet fluorescent materials, activated with cerium, of different compositions including Y and Al. With this configuration, light of desired color can be emitted by controlling the emission spectrum of the phosphor according to the property (wavelength of emitted light) of the light emitting component.
Further in the light emitting device of the present invention, in order to have light of a specified wavelength emitted by the light emitting device, it is preferable that the phosphor contains two or more fluorescent materials of different compositions represented by general formula (Re1xe2x88x92rSmr)3(Al1xe2x88x92sGas)5O12:Ce, where 0xe2x89xa6r less than 1 and 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y and Gd.
Also in the light emitting device of the present invention, in order to control the wavelength of emitted light, the phosphor may contain a first fluorescent material represented by general formula Y3(Al1xe2x88x92sGas)5O12:Ce and a second fluorescent material represented by general formula Re3Al5O12:Ce, where 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y, Gd and La.
Also in the light emitting device of the present invention, in order to control the wavelength of emitted light, the phosphor may be an yttrium-aluminum-garnet fluorescent material containing a first fluorescent material and a second fluorescent material, with different parts of each yttrium being substituted with gadolinium.
Further in the light emitting device of the present invention, it is preferable that main emission peak of the light emitting component is set within the range from 400 nm to 530 nm and main emission wavelength of the phosphor is set to be longer than the main emission peak of the light emitting component. This makes it possible to efficiently emit white light.
Further in the light emitting device of the present invention, it is preferable that the light emitting layer of the light emitting component contains a gallium nitride semiconductor which contains In, and the phosphor is an yttrium-aluminum-garnet fluorescent material wherein a part of Al in the yttrium-aluminum-garnet fluorescent is substituted by Ga so that the proportion of Ga:Al is within the range from 1:1 to 4:6 and a part of Y in the yttrium-aluminum-garnet fluorescent is substituted by Gd so that the proportion of Y:Gd is within the range from 4:1 to 2:3. Absorption spectrum of the phosphor which is controlled as described above shows good agreement with that of light emitted by the light emitting component which contains gallium nitride semiconductor including In as the light emitting layer, and is capable of improving the conversion efficiency (light emission efficiency). Also the light, generated by mixing blue light emitted by the light emitting component and fluorescent light of the fluorescent material, is a white light of good color rendering and, in this regard, an excellent light emitting device can be provided.
The light emitting device according to one embodiment of the present invention comprises a substantially rectangular optical guide plate provided with the light emitting component mounted on one side face thereof via the phosphor and surfaces of which except for one principal surface are substantially covered with a reflective material, where in a light emitted by the light emitting component is turned into a planar light by the phosphor and the optical guide plate and to be an output from the principal surface of the optical guide plate.
The light emitting device according to another embodiment of the present invention has a substantially rectangular optical guide plate, which is provided with the light emitting component mounted on one side face thereof and the phosphor installed on one principal surface with surfaces thereof and except for the principal surface being substantially covered with a reflective material, wherein a light emitted by the light emitting component is turned into a planar light by the optical guide plate and the phosphor, to be an output from the principal surface of the optical guide plate.
The LED display device according to the present invention has an LED display device comprising the light emitting devices of the present invention arranged in a matrix and a drive circuit which drives the LED display device according to display data which is input thereto. This configuration makes it possible to provide a relatively low-priced LED display device which is capable of high-definition display with less color unevenness due to the viewing angle.
The light emitting diode according to one embodiment of the present invention comprises:
a mount lead having a cup and a lead;
an LED chip mounted in the cup of the mount lead with one of electrodes being electrically connected to the mount lead;
a transparent coating material filling the cup to cover the LED chip; and
a light emitting diode having a molding material which covers the LED chip covered with the coating material including the cup of the mount lead, the inner lead and another electrode of the LED chip, wherein
the LED chip is a nitride compound semiconductor and the coating material contains at least one element selected from the group consisting of Y, Lu, Sc, La, Gd and Sm, at least one element selected from the group consisting of Al, Ga and In and a phosphor made of garnet fluorescent material activated with cerium.
The phosphor used in the light emitting diode of the present invention preferably contains an yttrium-aluminum-garnet fluorescent material that contains Y and Al.
In the light emitting diode of the present invention, the phosphor may be a fluorescent material represented by a general formula (Rexe2x88x92rSmr)3(Al1xe2x88x92sGas)5O12:Ce, where 0xe2x89xa6r less than 1 and 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y and Gd.
Also in the light emitting diode of the present invention, a fluorescent material represented by a general formula (Y1xe2x88x92pxe2x88x92qxe2x88x92rGdpCeqSmr)3(Al1xe2x88x92sGas)5O12 may be used as the phosphor, where 0xe2x89xa6pxe2x89xa60.8, 0.003xe2x89xa6qxe2x89xa60.2, 0.0003xe2x89xa6rxe2x89xa60.08 and 0xe2x89xa6sxe2x89xa61.
In the light emitting diode of the present invention, the phosphor preferably contain two or more yttrium-aluminum-garnet fluorescent materials, activated with cerium, of different compositions including Y and Al, in order to control the emitted light to a desired wavelength.
In the light emitting diode of the present invention, similarly, two or more fluorescent materials of different compositions represented by a general formula (Re1xe2x88x92rSmr)3(Al1xe2x88x92sGas)5O12:Ce, where 0xe2x89xa6r less than 1 and 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y and Gd may be used as the phosphor in order to control the emitted light to a desired wavelength.
In the light emitting diode of the present invention, similarly, a first fluorescent material represented by a general formula Y3(Al1xe2x88x92sGas)5O12:Ce and a second fluorescent material represented by a general formula Re3Al5O12:Ce, may be used as the phosphor where 0xe2x89xa6sxe2x89xa61 and Re is at least one selected from Y, Gd and La, in order to control the emitted light to a desired wavelength.
In the light emitting diode of the present invention, similarly, yttrium-aluminum-garnet fluorescent material a first fluorescent material and a second fluorescent material may be used wherein a part of yttrium in the first and second fluorescent materials is substituted with gadolinium to different degrees of substitution as the phosphor, in order to control the emitted light to a desired wavelength.
Generally, a fluorescent material which absorbs light of a short wavelength and emits light of a long wavelength has higher efficiency than a fluorescent material which absorbs light of a long wavelength and emits light of a short wavelength. It is preferable to use a light emitting component which emits visible light than a light emitting component which emits ultraviolet light that degrades resin (molding material, coating material, etc.) Thus for the light emitting diode of the present invention, for the purpose of improving the light emitting efficiency and ensure long life, it is preferable that main emission peak of the light emitting component be set within a relatively short wavelength range of 400 nm to 530 nm in the visible light region, and main emission wavelength of the phosphor be set to be longer than the main emission peak of the light emitting component. With this arrangement, because light converted by the fluorescent material has longer wavelength than that of light emitted by the light emitting component, it will not be absorbed by the light emitting component even when the light emitting component is irradiated with light which has been reflected and converted by the fluorescent material (since the energy of the converted light is less than the band gap energy). Thus the light which has been reflected by the fluorescent material or the like is reflected by the cup wherein the light emitting component is mounted, making higher efficiency of emission possible.